Distributed storage network data revision control

ABSTRACT

Multiple revisions of an encoded data slice are generated, with each revision having the same slice name. Each of the data slices represents the same original data portion, but each is encoded so that no single data slice can be used to reconstruct the original data portion. Appropriate revision numbers are associated with each encoded data slice, and the encoded data slices and associated revision numbers are transmitted for storage in selected storage units of a distributed storage network. If write confirmations are received from at least a write threshold number of storage units, a commit command is transmitted so that the most recently written data slices will be available for access. After a commit command is issued, a current directory used to access the encoded data slices can be sliced, encoded, and stored in the same way as the data slices.

CROSS REFERENCE TO RELATED PATENTS

The present U.S. Utility patent application claims priority pursuant to35 U.S.C. §119(e) to U.S. Provisional Application Ser. No. 61/256,226,entitled “DISTRIBUTED STORAGE NETWORK DATA REVISION CONTROL,” (AttorneyDocket No. CS089), filed Oct. 29, 2009, pending, which is herebyincorporated herein by reference in its entirety and made part of thepresent U.S. Utility patent application for all purposes.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

NOT APPLICABLE

INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC

NOT APPLICABLE

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

This invention relates generally to computing systems and moreparticularly to data storage solutions within such computing systems.

2. Description of Related Art

Computers are known to communicate, process, and store data. Suchcomputers range from wireless smart phones to data centers that supportmillions of web searches, stock trades, or on-line purchases every day.In general, a computing system generates data and/or manipulates datafrom one form into another. For instance, an image sensor of thecomputing system generates raw picture data and, using an imagecompression program (e.g., JPEG, MPEG, etc.), the computing systemmanipulates the raw picture data into a standardized compressed image.

With continued advances in processing speed and communication speed,computers are capable of processing real time multimedia data forapplications ranging from simple voice communications to streaming highdefinition video. As such, general-purpose information appliances arereplacing purpose-built communications devices (e.g., a telephone). Forexample, smart phones can support telephony communications but they arealso capable of text messaging and accessing the internet to performfunctions including email, web browsing, remote applications access, andmedia communications (e.g., telephony voice, image transfer, musicfiles, video files, real time video streaming. etc.).

Each type of computer is constructed and operates in accordance with oneor more communication, processing, and storage standards. As a result ofstandardization and with advances in technology, more and moreinformation content is being converted into digital formats. Forexample, more digital cameras are now being sold than film cameras, thusproducing more digital pictures. As another example, web-basedprogramming is becoming an alternative to over the air televisionbroadcasts and/or cable broadcasts. As further examples, papers, books,video entertainment, home video, etc. are now being stored digitally,which increases the demand on the storage function of computers.

A typical computer storage system includes one or more memory devicesaligned with the needs of the various operational aspects of thecomputer's processing and communication functions. Generally, theimmediacy of access dictates what type of memory device is used. Forexample, random access memory (RAM) memory can be accessed in any randomorder with a constant response time, thus it is typically used for cachememory and main memory. By contrast, memory device technologies thatrequire physical movement such as magnetic disks, tapes, and opticaldiscs, have a variable response time as the physical movement can takelonger than the data transfer, thus they are typically used forsecondary memory (e.g., hard drive, backup memory, etc.).

A computer's storage system will be compliant with one or more computerstorage standards that include, but are not limited to, network filesystem (NFS), flash file system (FFS), disk file system (DFS), smallcomputer system interface (SCSI), internet small computer systeminterface (iSCSI), file transfer protocol (FTP), and web-baseddistributed authoring and versioning (WebDAV). These standards specifythe data storage format (e.g., files, data objects, data blocks,directories, etc.) and interfacing between the computer's processingfunction and its storage system, which is a primary function of thecomputer's memory controller.

Despite the standardization of the computer and its storage system,memory devices fail; especially commercial grade memory devices thatutilize technologies incorporating physical movement (e.g., a discdrive). For example, it is fairly common for a disc drive to routinelysuffer from bit level corruption and to completely fail after threeyears of use. One solution is to a higher-grade disc drive, which addssignificant cost to a computer.

Another solution is to utilize multiple levels of redundant disc drivesto replicate the data into two or more copies. One such redundant driveapproach is called redundant array of independent discs (RAID). In aRAID device, a RAID controller adds parity data to the original databefore storing it across the array. The parity data is calculated fromthe original data such that the failure of a disc will not result in theloss of the original data. For example, RAID 5 uses three discs toprotect data from the failure of a single disc. The parity data, andassociated redundancy overhead data, reduces the storage capacity ofthree independent discs by one third (e.g., n−1=capacity). RAID 6 canrecover from a loss of two discs and requires a minimum of four discswith a storage capacity of n−2.

While RAID addresses the memory device failure issue, it is not withoutits own failures issues that affect its effectiveness, efficiency andsecurity. For instance, as more discs are added to the array, theprobability of a disc failure increases, which increases the demand formaintenance. For example, when a disc fails, it needs to be manuallyreplaced before another disc fails and the data stored in the RAIDdevice is lost. To reduce the risk of data loss, data on a RAID deviceis typically copied on to one or more other RAID devices. While thisaddresses the loss of data issue, it raises a security issue sincemultiple copies of data are available, which increases the chances ofunauthorized access. Further, as the amount of data being stored grows,the overhead of RAID devices becomes a non-trivial efficiency issue.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic block diagram of an embodiment of a computingsystem in accordance with the invention;

FIG. 2 is a schematic block diagram of an embodiment of a computing corein accordance with the invention;

FIG. 3 is a schematic block diagram of an embodiment of a distributedstorage processing unit in accordance with the invention;

FIG. 4 is a schematic block diagram of an embodiment of a grid module inaccordance with the invention;

FIG. 5 is a diagram of an example embodiment of error coded data slicecreation in accordance with the invention;

FIG. 6 is a flowchart illustrating the determination of a data revisionnumber;

FIG. 7 is a flowchart illustrating the retrieving of like revision data;

FIG. 8 is a flowchart illustrating the storing of data;

FIG. 9 is another flowchart illustrating the storing of data;

FIG. 10 is another flowchart illustrating the storing of data;

FIG. 11 is a flowchart illustrating the deleting of data; and

FIG. 12 is a flowchart illustrating the retrieving of data

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic block diagram of a computing system 10 thatincludes one or more of a first type of user devices 12, one or more ofa second type of user devices 14, at least one distributed storage (DS)processing unit 16, at least one DS managing unit 18, at least onestorage integrity processing unit 20, and a distributed storage network(DSN) memory 22 coupled via a network 24. The network 24 may include oneor more wireless and/or wire lined communication systems; one or moreprivate intranet systems and/or public internet systems; and/or one ormore local area networks (LAN) and/or wide area networks (WAN).

The DSN memory 22 includes a plurality of distributed storage (DS) units36 for storing data of the system. Each of the DS units 36 includes aprocessing module and memory and may be located at a geographicallydifferent site than the other DS units (e.g., one in Chicago, one inMilwaukee, etc.). The processing module may be a single processingdevice or a plurality of processing devices. Such a processing devicemay be a microprocessor, micro-controller, digital signal processor,microcomputer, central processing unit, field programmable gate array,programmable logic device, state machine, logic circuitry, analogcircuitry, digital circuitry, and/or any device that manipulates signals(analog and/or digital) based on hard coding of the circuitry and/oroperational instructions. The processing module may have an associatedmemory and/or memory element, which may be a single memory device, aplurality of memory devices, and/or embedded circuitry of the processingmodule. Such a memory device may be a read-only memory, random accessmemory, volatile memory, non-volatile memory, static memory, dynamicmemory, flash memory, cache memory, and/or any device that storesdigital information. Note that if the processing module includes morethan one processing device, the processing devices may be centrallylocated (e.g., directly coupled together via a wired and/or wireless busstructure) or may be distributedly located (e.g., cloud computing viaindirect coupling via a local area network and/or a wide area network).Further note that when the processing module implements one or more ofits functions via a state machine, analog circuitry, digital circuitry,and/or logic circuitry, the memory and/or memory element storing thecorresponding operational instructions may be embedded within, orexternal to, the circuitry comprising the state machine, analogcircuitry, digital circuitry, and/or logic circuitry. Still further notethat, the memory element stores, and the processing module executes,hard coded and/or operational instructions corresponding to at leastsome of the steps and/or functions illustrated in FIGS. 1-12.

Each of the user devices 12-14, the DS processing unit 16, the DSmanaging unit 18, and the storage integrity processing unit 20 may be aportable computing device (e.g., a social networking device, a gamingdevice, a cell phone, a smart phone, a personal digital assistant, adigital music player, a digital video player, a laptop computer, ahandheld computer, a video game controller, and/or any other portabledevice that includes a computing core) and/or a fixed computing device(e.g., a personal computer, a computer server, a cable set-top box, asatellite receiver, a television set, a printer, a fax machine, homeentertainment equipment, a video game console, and/or any type of homeor office computing equipment). Such a portable or fixed computingdevice includes a computing core 26 and one or more interfaces 30, 32,and/or 33. An embodiment of the computing core 26 will be described withreference to FIG. 2.

With respect to the interfaces, each of the interfaces 30, 32, and 33includes software and/or hardware to support one or more communicationlinks via the network 24 and/or directly. For example, interfaces 30support a communication link (wired, wireless, direct, via a LAN, viathe network 24, etc.) between the first type of user device 14 and theDS processing unit 16. As another example, DSN interface 32 supports aplurality of communication links via the network 24 between the DSNmemory 22 and the DS processing unit 16, the first type of user device12, and/or the storage integrity processing unit 20. As yet anotherexample, interface 33 supports a communication link between the DSmanaging unit 18 and any one of the other devices and/or units 12, 14,16, 20, and/or 22 via the network 24.

In general and with respect to data storage, the system 10 supportsthree primary functions: distributed network data storage management,distributed data storage and retrieval, and data storage integrityverification. In accordance with these three primary functions, data canbe distributedly stored in a plurality of physically different locationsand subsequently retrieved in a reliable and secure manner regardless offailures of individual storage devices, failures of network equipment,the duration of storage, the amount of data being stored, attempts athacking the data, etc.

The DS managing unit 18 performs distributed network data storagemanagement functions, which include establishing distributed datastorage parameters, performing network operations, performing networkadministration, and/or performing network maintenance. The DS managingunit 18 establishes the distributed data storage parameters (e.g.,allocation of virtual DSN memory space, distributed storage parameters,security parameters, billing information, user profile information,etc.) for one or more of the user devices 12-14 (e.g., established forindividual devices, established for a user group of devices, establishedfor public access by the user devices, etc.). For example, the DSmanaging unit 18 coordinates the creation of a vault (e.g., a virtualmemory block) within the DSN memory 22 for a user device (for a group ofdevices, or for public access). The DS managing unit 18 also determinesthe distributed data storage parameters for the vault. In particular,the DS managing unit 18 determines a number of slices (e.g., the numberthat a data segment of a data file and/or data block is partitioned intofor distributed storage) and a read threshold value (e.g., the minimumnumber of slices required to reconstruct the data segment).

As another example, the DS managing module 18 creates and stores,locally or within the DSN memory 22, user profile information. The userprofile information includes one or more of authentication information,permissions, and/or the security parameters. The security parameters mayinclude one or more of encryption/decryption scheme, one or moreencryption keys, key generation scheme, and data encoding/decodingscheme.

As yet another example, the DS managing unit 18 creates billinginformation for a particular user, user group, vault access, publicvault access, etc. For instance, the DS managing unit 18 tracks thenumber of times user accesses a private vault and/or public vaults,which can be used to generate a per-access bill. In another instance,the DS managing unit 18 tracks the amount of data stored and/orretrieved by a user device and/or a user group, which can be used togenerate a per-data-amount bill.

The DS managing unit 18 also performs network operations, networkadministration, and/or network maintenance. As at least part ofperforming the network operations and/or administration, the DS managingunit 18 monitors performance of the devices and/or units of the system10 for potential failures, determines the devices and/or unit'sactivation status, determines the devices' and/or units' loading, andany other system level operation that affects the performance level ofthe system 10. For example, the DS managing unit 18 receives andaggregates network management alarms, alerts, errors, statusinformation, performance information, and messages from the devices12-14 and/or the units 16, 20, 22. For example, the DS managing unit 18receives a simple network management protocol (SNMP) message regardingthe status of the DS processing unit 16.

The DS managing unit 18 performs the network maintenance by identifyingequipment within the system 10 that needs replacing, upgrading,repairing, and/or expanding. For example, the DS managing unit 18determines that the DSN memory 22 needs more DS units 36 or that one ormore of the DS units 36 needs updating.

The second primary function (i.e., distributed data storage andretrieval) begins and ends with a user device 12-14. For instance, if asecond type of user device 14 has a data file 38 and/or data block 40 tostore in the DSN memory 22, it send the data file 38 and/or data block40 to the DS processing unit 16 via its interface 30. As will bedescribed in greater detail with reference to FIG. 2, the interface 30functions to mimic a conventional operating system (OS) file systeminterface (e.g., network file system (NFS), flash file system (FFS),disk file system (DFS), file transfer protocol (FTP), web-baseddistributed authoring and versioning (WebDAV), etc.) and/or a blockmemory interface (e.g., small computer system interface (SCSI), internetsmall computer system interface (iSCSI), etc.). In addition, theinterface 30 may attach a user identification code (ID) to the data file38 and/or data block 40.

The DS processing unit 16 receives the data file 38 and/or data block 40via its interface 30 and performs a distributed storage (DS) process 34thereon (e.g., an error coding dispersal storage function). The DSprocessing 34 begins by partitioning the data file 38 and/or data block40 into one or more data segments, which is represented as Y datasegments. For example, the DS processing 34 may partition the data file38 and/or data block 40 into a fixed byte size segment (e.g., 2¹ to2^(n) bytes, where n=>2) or a variable byte size (e.g., change byte sizefrom segment to segment, or from groups of segments to groups ofsegments, etc.).

For each of the Y data segments, the DS processing 34 error encodes(e.g., forward error correction (FEC), information dispersal algorithm,or error correction coding) and slices (or slices then error encodes)the data segment into a plurality of error coded (EC) data slices 42-48,which is represented as X slices per data segment. The number of slices(X) per segment, which corresponds to a number of pillars n, is set inaccordance with the distributed data storage parameters and the errorcoding scheme. For example, if a Reed-Solomon (or other FEC scheme) isused in an n/k system, then a data segment is divided into n slices,where k number of slices is needed to reconstruct the original data(i.e., k is the threshold). As a few specific examples, the n/k factormay be 5/3; 6/4; 8/6; 8/5; 16/10.

For each slice 42-48, the DS processing unit 16 creates a unique slicename and associates it with, or appends it to, the corresponding slice42-48. The slice name includes universal DSN memory addressing routinginformation (e.g., virtual memory addresses in the DSN memory 22) anduser-specific information (e.g., user ID, file name, data blockidentifier, etc.).

The DS processing unit 16 transmits the plurality of EC slices 42-48 toa plurality of DS units 36 of the DSN memory 22 via the DSN interface 32and the network 24. The DSN interface 32 formats each of the slices fortransmission via the network 24. For example, the DSN interface 32 mayutilize an internet protocol (e.g., TCP/IP, etc.) to packetize theslices 42-48 for transmission via the network 24.

The number of DS units 36 receiving the slices 42-48 is dependent on thedistributed data storage parameters established by the DS managing unit18. For example, the DS managing unit 18 may indicate that each slice isto be stored in a different DS unit 36. As another example, the DSmanaging unit 18 may indicate that like slice numbers of different datasegments are to be stored in the same DS unit 36. For example, the firstslice of each of the data segments is to be stored in a first DS unit36, the second slice of each of the data segments is to be stored in asecond DS unit 36, etc. In this manner, the data is encoded anddistributedly stored at physically diverse locations to improved datastorage integrity and security. Further examples of encoding the datasegments will be provided with reference to one or more of FIGS. 2-12.

Each DS unit 36 that receives a slice 42-48 for storage translates thevirtual DSN memory address of the slice into a local physical addressfor storage. Accordingly, each DS unit 36 maintains a virtual tophysical memory mapping to assist in the storage and retrieval of data.

The first type of user device 12 performs a similar function to storedata in the DSN memory 22 with the exception that it includes the DSprocessing. As such, the device 12 encodes and slices the data fileand/or data block it has to store. The device then transmits the slices11 to the DSN memory via its DSN interface 32 and the network 24.

For a second type of user device 14 to retrieve a data file or datablock from memory, it issues a read command via its interface 30 to theDS processing unit 16. The DS processing unit 16 performs the DSprocessing 34 to identify the DS units 36 storing the slices of the datafile and/or data block based on the read command. The DS processing unit16 may also communicate with the DS managing unit 18 to verify that theuser device 14 is authorized to access the requested data.

Assuming that the user device is authorized to access the requesteddata, the DS processing unit 16 issues slice read commands to at least athreshold number of the DS units 36 storing the requested data (e.g., toat least 10 DS units for a 16/10 error coding scheme). Each of the DSunits 36 receiving the slice read command, verifies the command,accesses its virtual to physical memory mapping, retrieves the requestedslice, or slices, and transmits it to the DS processing unit 16.

Once the DS processing unit 16 has received a read threshold number ofslices for a data segment, it performs an error decoding function andde-slicing to reconstruct the data segment. When Y number of datasegments has been reconstructed, the DS processing unit 16 provides thedata file 38 and/or data block 40 to the user device 14. Note that thefirst type of user device 12 performs a similar process to retrieve adata file and/or data block.

The storage integrity processing unit 20 performs the third primaryfunction of data storage integrity verification. In general, the storageintegrity processing unit 20 periodically retrieves slices 45, and/orslice names, of a data file or data block of a user device to verifythat one or more slices have not been corrupted or lost (e.g., the DSunit failed). The retrieval process mimics the read process previouslydescribed.

If the storage integrity processing unit 20 determines that one or moreslices is corrupted or lost, it rebuilds the corrupted or lost slice(s)in accordance with the error coding scheme. The storage integrityprocessing unit 20 stores the rebuild slice, or slices, in theappropriate DS unit(s) 36 in a manner that mimics the write processpreviously described.

FIG. 2 is a schematic block diagram of an embodiment of a computing core26 that includes a processing module 50, a memory controller 52, mainmemory 54, a video graphics processing unit 55, an input/output (IO)controller 56, a peripheral component interconnect (PCI) interface 58,at least one IO device interface module 62, a read only memory (ROM)basic input output system (BIOS) 64, and one or more memory interfacemodules. The memory interface module(s) includes one or more of auniversal serial bus (USB) interface module 66, a host bus adapter (HBA)interface module 68, a network interface module 70, a flash interfacemodule 72, a hard drive interface module 74, and a DSN interface module76. Note the DSN interface module 76 and/or the network interface module70 may function as the interface 30 of the user device 14 of FIG. 1.Further note that the IO device interface module 62 and/or the memoryinterface modules may be collectively or individually referred to as IOports.

The processing module 50 may be a single processing device or aplurality of processing devices. Such a processing device may be amicroprocessor, micro-controller, digital signal processor,microcomputer, central processing unit, field programmable gate array,programmable logic device, state machine, logic circuitry, analogcircuitry, digital circuitry, and/or any device that manipulates signals(analog and/or digital) based on hard coding of the circuitry and/oroperational instructions. The processing module 50 may have anassociated memory and/or memory element, which may be a single memorydevice, a plurality of memory devices, and/or embedded circuitry of theprocessing module 50. Such a memory device may be a read-only memory,random access memory, volatile memory, non-volatile memory, staticmemory, dynamic memory, flash memory, cache memory, and/or any devicethat stores digital information. Note that if the processing module 50includes more than one processing device, the processing devices may becentrally located (e.g., directly coupled together via a wired and/orwireless bus structure) or may be distributedly located (e.g., cloudcomputing via indirect coupling via a local area network and/or a widearea network). Further note that when the processing module 50implements one or more of its functions via a state machine, analogcircuitry, digital circuitry, and/or logic circuitry, the memory and/ormemory element storing the corresponding operational instructions may beembedded within, or external to, the circuitry comprising the statemachine, analog circuitry, digital circuitry, and/or logic circuitry.Still further note that, the memory element stores, and the processingmodule 50 executes, hard coded and/or operational instructionscorresponding to at least some of the steps and/or functions illustratedin FIGS. 1-12.

FIG. 3 is a schematic block diagram of an embodiment of a dispersedstorage (DS) processing module 34 of user device 12 and/or of the DSprocessing unit 16. The DS processing module 34 includes a gatewaymodule 78, an access module 80, a grid module 82, and a storage module84. The DS processing module 34 may also include an interface 30 and theDSnet interface 32 or the interfaces 68 and/or 70 may be part of user 12or of the DS processing unit 14. The DS processing module 34 may furtherinclude a bypass/feedback path between the storage module 84 to thegateway module 78. Note that the modules 78-84 of the DS processingmodule 34 may be in a single unit or distributed across multiple units.

In an example of storing data, the gateway module 78 receives anincoming data object that includes a user ID field 86, an object namefield 88, and the data field 40 and may also receive correspondinginformation that includes a process identifier (e.g., an internalprocess/application ID), metadata, a file system directory, a blocknumber, a transaction message, a user device identity (ID), a dataobject identifier, a source name, and/or user information. The gatewaymodule 78 authenticates the user associated with the data object byverifying the user ID 86 with the managing unit 18 and/or anotherauthenticating unit.

When the user is authenticated, the gateway module 78 obtains userinformation from the management unit 18, the user device, and/or theother authenticating unit. The user information includes a vaultidentifier, operational parameters, and user attributes (e.g., userdata, billing information, etc.). A vault identifier identifies a vault,which is a virtual memory space that maps to a set of DS storage units36. For example, vault 1 (i.e., user 1's DSN memory space) includeseight DS storage units (X=8 wide) and vault 2 (i.e., user 2's DSN memoryspace) includes sixteen DS storage units (X=16 wide). The operationalparameters may include an error coding algorithm, the width n (number ofpillars X or slices per segment for this vault), a read threshold T, awrite threshold, an encryption algorithm, a slicing parameter, acompression algorithm, an integrity check method, caching settings,parallelism settings, and/or other parameters that may be used to accessthe DSN memory layer.

The gateway module 78 uses the user information to assign a source name35 to the data. For instance, the gateway module 60 determines thesource name 35 of the data object 40 based on the vault identifier andthe data object. For example, the source name may contain a fileidentifier (ID), a vault generation number, a reserved field, and avault identifier (ID). As another example, the gateway module 78 maygenerate the file ID based on a hash function of the data object 40.Note that the gateway module 78 may also perform message conversion,protocol conversion, electrical conversion, optical conversion, accesscontrol, user identification, user information retrieval, trafficmonitoring, statistics generation, configuration, management, and/orsource name determination.

The access module 80 receives the data object 40 and creates a series ofdata segments 1 through Y 90-92 in accordance with a data storageprotocol (e.g., file storage system, a block storage system, and/or anaggregated block storage system). The number of segments Y may be chosenor randomly assigned based on a selected segment size and the size ofthe data object. For example, if the number of segments is chosen to bea fixed number, then the size of the segments varies as a function ofthe size of the data object. For instance, if the data object is animage file of 4,194,304 eight bit bytes (e.g., 33,554,432 bits) and thenumber of segments Y=131,072, then each segment is 256 bits or 32 bytes.As another example, if segment sized is fixed, then the number ofsegments Y varies based on the size of data object. For instance, if thedata object is an image file of 4,194,304 bytes and the fixed size ofeach segment is 4,096 bytes, the then number of segments Y=1,024. Notethat each segment is associated with the same source name.

The grid module 82 receives the data segments and may manipulate (e.g.,compression, encryption, cyclic redundancy check (CRC), etc.) each ofthe data segments before performing an error coding function of theerror coding dispersal storage function to produce a pre-manipulateddata segment. After manipulating a data segment, if applicable, the gridmodule 82 error encodes (e.g., Reed-Solomon, Convolution encoding,Trellis encoding, etc.) the data segment or manipulated data segmentinto X error coded data slices 42-44.

The value X, or the number of pillars (e.g., X=16), is chosen as aparameter of the error coding dispersal storage function. Otherparameters of the error coding dispersal function include a readthreshold T, a write threshold W, etc. The read threshold (e.g., T=10,when X=16) corresponds to the minimum number of error-free error codeddata slices required to reconstruct the data segment. In other words,the DS processing module 34 can compensate for X−T (e.g., 16−10=6)missing error coded data slices per data segment. The write threshold Wcorresponds to a minimum number of DS storage units that acknowledgeproper storage of their respective data slices before the DS processingmodule indicates proper storage of the encoded data segment. Note thatthe write threshold is greater than or equal to the read threshold for agiven number of pillars (X).

For each data slice of a data segment, the grid module 82 generates aunique slice name 37 and attaches it thereto. The slice name 37 includesa universal routing information field and a vault specific field and maybe 48 bytes (e.g., 24 bytes for each of the universal routinginformation field and the vault specific field). As illustrated, theuniversal routing information field includes a slice index, a vault ID,a vault generation, and a reserved field. The slice index is based onthe pillar number and the vault ID and, as such, is unique for eachpillar (e.g., slices of the same pillar for the same vault for anysegment will share the same slice index). The vault specific fieldincludes a data name, which includes a file ID and a segment number(e.g., a sequential numbering of data segments 1-Y of a simple dataobject or a data block number).

Prior to outputting the error coded data slices of a data segment, thegrid module may perform post-slice manipulation on the slices. Ifenabled, the manipulation includes slice level compression, encryption,CRC, addressing, tagging, and/or other manipulation to improve theeffectiveness of the computing system.

When the error coded data slices of a data segment are ready to beoutputted, the grid module 82 determines which of the DS storage units36 will store the EC data slices based on a dispersed storage memorymapping associated with the user's vault and/or DS storage unit 36attributes. The DS storage unit attributes may include availability,self-selection, performance history, link speed, link latency,ownership, available DSN memory, domain, cost, a prioritization scheme,a centralized selection message from another source, a lookup table,data ownership, and/or any other factor to optimize the operation of thecomputing system. Note that the number of DS storage units 36 is equalto or greater than the number of pillars (e.g., X) so that no more thanone error coded data slice of the same data segment is stored on thesame DS storage unit 36. Further note that EC data slices of the samepillar number but of different segments (e.g., EC data slice 1 of datasegment 1 and EC data slice 1 of data segment 2) may be stored on thesame or different DS storage units 36.

The storage module 84 performs an integrity check on the outboundencoded data slices and, when successful, identifies a plurality of DSstorage units based on information provided by the grid module. Thestorage module then outputs the encoded data slices 1 through X of eachsegment 1 through Y to the DS storage units. Each of the DS storageunits 36 stores its EC data slice(s) and maintains a local virtual DSNaddress to physical location table to convert the virtual DSN address ofthe EC data slice(s) into physical storage addresses.

In an example of a read operation, the user device 12 and/or 14 sends aread request to the DS processing unit 14, which authenticates therequest. When the request is authentic, the DS processing unit 14 sendsa read message to each of the DS storage units 36 storing slices of thedata object being read. The slices are received via the DSnet interface32 and processed by the storage module 84, which performs a parity checkand provides the slices to the grid module 82 when the parity check wassuccessful. The grid module 82 decodes the slices in accordance with theerror coding dispersal storage function to reconstruct the data segment.The access module 80 reconstructs the data object from the data segmentsand the gateway module 78 formats the data object for transmission tothe user device.

FIG. 4 is a schematic block diagram of an embodiment of a grid module 82that includes a control unit 73, a pre-slice manipulator 75, an encoder77, a slicer 79, a post-slice manipulator 81, a pre-slice de-manipulator83, a decoder 85, a de-slicer 87, and/or a post-slice de-manipulator 89.Note that the control unit 73 may be partially or completely external tothe grid module 82. For example, the control unit 73 may be part of thecomputing core at a remote location, part of a user device, part of theDS managing unit 18, or distributed amongst one or more DS storageunits.

In an example of write operation, the pre-slice manipulator 75 receivesa data segment 90-92 and a write instruction from an authorized userdevice. The pre-slice manipulator 75 determines if pre-manipulation ofthe data segment 90-92 is required and, if so, what type. The pre-slicemanipulator 75 may make the determination independently or based oninstructions from the control unit 73, where the determination is basedon a computing system-wide predetermination, a table lookup, vaultparameters associated with the user identification, the type of data,security requirements, available DSN memory, performance requirements,and/or other metadata.

Once a positive determination is made, the pre-slice manipulator 75manipulates the data segment 90-92 in accordance with the type ofmanipulation. For example, the type of manipulation may be compression(e.g., Lempel-Ziv-Welch, Huffman, Golomb, fractal, wavelet, etc.),signatures (e.g., Digital Signature Algorithm (DSA), Elliptic Curve DSA,Secure Hash Algorithm, etc.), watermarking, tagging, encryption (e.g.,Data Encryption Standard, Advanced Encryption Standard, etc.), addingmetadata (e.g., time/date stamping, user information, file type, etc.),cyclic redundancy check (e.g., CRC32), and/or other data manipulationsto produce the pre-manipulated data segment.

The encoder 77 encodes the pre-manipulated data segment 92 using aforward error correction (FEC) encoder (and/or other type of erasurecoding and/or error coding) to produce an encoded data segment 94. Theencoder 77 determines which forward error correction algorithm to usebased on a predetermination associated with the user's vault, a timebased algorithm, user direction, DS managing unit direction, controlunit direction, as a function of the data type, as a function of thedata segment 92 metadata, and/or any other factor to determine algorithmtype. The forward error correction algorithm may be Golay,Multidimensional parity, Reed-Solomon, Hamming, Bose Ray ChauduriHocquenghem (BCH), Cauchy-Reed-Solomon, or any other FEC encoder. Notethat the encoder 77 may use a different encoding algorithm for each datasegment 92, the same encoding algorithm for the data segments 92 of adata object, or a combination thereof.

The encoded data segment 94 is of greater size than the data segment 92by the overhead rate of the encoding algorithm by a factor of X/T, whereX is the width or number of slices, and T is the read threshold. In thisregard, the corresponding decoding process can accommodate at most X-Tmissing EC data slices and still recreate the data segment 92. Forexample, if X=16 and T=10, then the data segment 92 will be recoverableas long as 10 or more EC data slices per segment are not corrupted.

The slicer 79 transforms the encoded data segment 94 into EC data slicesin accordance with the slicing parameter from the vault for this userand/or data segment 92. For example, if the slicing parameter is X=16,then the slicer slices each encoded data segment 94 into 16 encodedslices.

The post-slice manipulator 81 performs, if enabled, post-manipulation onthe encoded slices to produce the EC data slices. If enabled, thepost-slice manipulator 81 determines the type of post-manipulation,which may be based on a computing system-wide predetermination,parameters in the vault for this user, a table lookup, the useridentification, the type of data, security requirements, available DSNmemory, performance requirements, control unit directed, and/or othermetadata. Note that the type of post-slice manipulation may includeslice level compression, signatures, encryption, CRC, addressing,watermarking, tagging, adding metadata, and/or other manipulation toimprove the effectiveness of the computing system.

In an example of a read operation, the post-slice de-manipulator 89receives at least a read threshold number of EC data slices and performsthe inverse function of the post-slice manipulator 81 to produce aplurality of encoded slices. The de-slicer 87 de-slices the encodedslices to produce an encoded data segment 94. The decoder 85 performsthe inverse function of the encoder 77 to recapture the data segment90-92. The pre-slice de-manipulator 83 performs the inverse function ofthe pre-slice manipulator 75 to recapture the data segment.

FIG. 5 is a diagram of an example of slicing an encoded data segment 94by the slicer 79. In this example, the encoded data segment includesthirty-two bits, but may include more or less bits. The slicer 79disperses the bits of the encoded data segment 94 across the EC dataslices in a pattern as shown. As such, each EC data slice does notinclude consecutive bits of the data segment 94 reducing the impact ofconsecutive bit failures on data recovery. For example, if EC data slice2 (which includes bits 1, 5, 9, 13, 17, 25, and 29) is unavailable(e.g., lost, inaccessible, or corrupted), the data segment can bereconstructed from the other EC data slices (e.g., 1, 3 and 4 for a readthreshold of 3 and a width of 4).

FIG. 6 is a flowchart illustrating the determination of a data revisionnumber where the DS processing determines the revision number andappends it to, or associates it with EC data slices being distributedlystored. The DS processing subsequent retrieval of EC data slicesverifies that the slices utilized to recreate the data segment have thesame appended revision number to improve data consistency and systemperformance. The retrieval method will be discussed in greater detailwith reference to FIG. 7.

The method 600 begins with the step 602 where the DS processing createsEC data slices for a data segment in accordance with the operationalparameters as previously discussed. As illustrated by block 604, the DSprocessing determines the revision number for the slices of the datasegment based on one or more of a timestamp, a random number, the uservault ID, the user ID, the data object ID, a hash of the data object,and/or a hash of the data object ID. For example, the revision numbermay be eight bytes and comprise a UNIX time timestamp and a randomnumber (e.g., to provide an improvement of a unique revision number whendata is stored at the same time).

As illustrated by block 606, the DS processing appends the revisionnumber to each pillar slice of the same data segment such that eachpillar slice of the same data segment has the same revision number. Inan embodiment, the DS processing appends the same revision number to allthe slices of all the data segments of the data object. In anotherembodiment, the DS processing appends the same revision number to allthe slices of each data segment but the revision numbers from datasegment to data segment of the data object are different.

As illustrated by block 608, the DS processing determines the DS unitsto send the slices to in accordance with the virtual DSN address tophysical location table for the user vault of the data object.

As illustrated by block 610, the DS processing sends a write command andslices with the appended revision number to the DS units such that theDS units will store the slices and send a write confirmation message tothe DS processing in response. Note that the slices are substantiallysent in parallel from the DS processing to the DS units via similar ordifferent portions of the network. Networks are known to fail from timeto time thus all of the DS units may not receive the slices. As a resultof network failures and other potential issues, the DS units may containslices with different revision numbers for the same data object and/ordata segment.

The DS processing receives the write confirmation message from the DSunits. The DS processing determines a write threshold (e.g., from theuser vault, a command, a predetermination) where the write threshold isthe minimum number of pillars required to store the unique slices of thesame data segment to meet the criteria of a favorable write sequence.The write threshold is less than the pillar width n and greater than theread threshold (discussed previously).

As illustrated by blocks 612 and 616, the DS processing determines ifthe number of received write confirmation messages is equal to orgreater than the write threshold. The determination may be based on oneor more of comparing the number of received write confirmations to thewrite threshold, a command, a predetermination, and/or a systemperformance indicator. The DS processing may continue to keep checkingfor new write confirmations when the DS processing determines that thenumber of received write confirmation messages is not equal to orgreater than the write threshold. As further illustrated by block 616,the DS processing may fail the write sequence if a predetermined periodof time expires before the DS processing determines that the number ofreceived write confirmation messages is equal to or greater than thewrite threshold.

As illustrated by blocks 614, the DS processing sends a write commitcommand to the DS units when the DS processing determines that thenumber of received write confirmation messages is equal to or greaterthan the write threshold. The DS unit makes the slice visible onsubsequent retrievals when the DS unit receives the write commit commandfor slices the DS unit has write confirmed.

The DS unit may request the slice be resent from the DS processing whenthe DS unit receives the write commit command for slices the DS unit hasnot write confirmed. The DS processing sends the write command andslices with the appended revision number to the DS units such that theDS units will store the slices and send a write confirmation message tothe DS processing in response when the DS processing receives the slicerequest from the DS unit.

FIG. 7 is a flowchart illustrating the retrieving of like revision datawhere the DS processing retrieves slices from the DS unit pillars andverifies that the slices have the same appended revision numbers toimprove data consistency.

According to method 700, at block 702 the DS processing determines theDS units (the pillars) to retrieve slices from in accordance with thevirtual DSN address to physical location table for the user vault of thedata object. As further illustrated by block 702, the DS processingsends a retrieve command message to the DS units where the messageincludes the slice name. The DS processing sends the retrieve command toat least a read threshold number of DS units. For example, the DSprocessing sends the retrieve command to ten DS units in a 16/10 DSNsystem. In another example, the DS processing sends the retrieve commandto twelve DS units in a 16/10 DSN system to provide better performance.The DS processing may create and temporarily save a list of DS unitsthat were sent the retrieve command such that the DS processing maychoose different DS units in a subsequent retrieval attempt if thepresent retrieval attempt fails. DS units send the slice and appendedrevision number corresponding the slice name to the DS processing whenthe DS unit receives the retrieval command message.

As illustrated by block 704, the DS processing receives the slices andappended revision number from the DS units. The DS processing determinesthe number of received slices by counting them. The DS processingdetermines the read threshold number for this vault by retrieving theread threshold number form the vault. The DS processing proceeds to thenext step when the DS processing determines that at least a readthreshold number of slices have been received from the DS units.

As illustrated by blocks 706 and 708, the DS processing determines ifthe appended revision numbers for the slices from each of the DS unitsare the same by comparing the revision numbers. Note that it is possiblefor some of the revision numbers to be different (e.g., as a result of afailure of a previous write sequence or some other DS unit failure).

As illustrated by block 710, the DS processing determines different DSunits to send retrieval commands and the DS processing sends theretrieve command to at least a read threshold number of DS units whenthe appended revision numbers for the slices from each of the current DSunits are not the same. The determination may be based on which DS unitswere already tried (saved previously) and which of those were in amajority where the majority had the same revision number. In such ascenario, the DS processing need only send a retrieval command messageto a smaller set of DS units that have not been tried yet. The methodbranches to the step of the DS processing receiving a read thresholdnumber of slices with appended revision numbers from the DS units.

As illustrated by block 712, the DS processing de-slices and decodes theslices to produce the data segment when the appended revision numbersfor the slices from each of the current DS units are the same.

FIG. 8 is a flowchart illustrating the storing of data where the DSprocessing utilizes a transaction process to improve data consistency.

The method 800 begins at block 802, where the DS processing receives adata object to store (e.g., from the user device). As illustrated byblock 804, the DS processing creates the slices in accordance with theoperational parameters and appends revision numbers created aspreviously discussed with reference to FIG. 6. The DS processingdetermines the DS units and sends a write command and the slices withthe appended revision numbers. The determination may be based on thevirtual DSN address to physical location table.

As further illustrated by block 804, the DS unit sends a write commandconfirmation message to the DS processing in response to receiving thewrite command. The DS processing receives the write command confirmationmessages, counts them, and determines if a write threshold number ofconfirmations has been received by comparing the count of received writecommand confirmation messages to the write threshold. The DS processingsends a write commit command to the DS units where the DS processingreceived a write command confirmation message. Note that now the newestrevision is successfully stored in the DSN.

In the next steps, the DS processing stores the directory in the DSNmemory where the directory links the user root file to the data objectto the slice name and revisions. In other words, the directory maps thedata object to locations of encoded data slices generated from the dataobjects. These locations may be virtual DSN addresses, such as a sourcename or slice name that is further translated, e.g., via a lookup in avirtual DSN address to physical location table, to the DS unitlocations, e.g., DS unit identifier.

As illustrated by block 808, the DS processing determines the currentdirectory (e.g., reading it in the DS processing file system, receivingit from the user device, etc.) and caches it locally in the DSprocessing. As shown at block 810, the DS processing creates slices forthe current directory and sends the slices with the write command and arevision number (e.g., the directory is assigned a revision number) tothe DS units associated with the user vault. After caching the currentdirectory, and before sending the directory slices to be stored, a newentry can be added to represent the directory slices. The DS unitsreceive the slices and write command and will process the write sequenceas will be discussed in greater with reference to FIG. 9.

As illustrated by blocks 812 and 814, the DS processing determines theresponse from the DS units where the response may be a write failure ora write success (e.g., a write command confirmation). The write failuremay result from one or more of the slice names were already write locked(e.g., an active write transaction was already in process), the DSprocessing determines that the number of write confirmations is belowthe write threshold, and/or the number of write confirmations with thesame revision numbers is below the write threshold.

As illustrated by block 814, the DS processing branches back to the stepof determining the directory when the DS processing determines theresponse from the DS units is a write failure.

As illustrated by block 816, the DS processing sends a write commitcommand to the DS units where the DS processing received successfulwrite confirmations when the DS processing determines the response fromthe DS units is a write success. Note that this step will activate therevision of the current directory.

FIG. 9 is another flowchart illustrating the storing of data where theDS unit processes write transactions in accordance with a transactionprocess to improve the consistency of stored data.

The method 900 begins at block 902, where the DS unit receives a writecommand, slice name, revision number, and slice for storage. The DS unitmay determine if this slice name is already in a write locked state(e.g., in an active write sequence) by a lookup. The DS unit may send awrite lock failure message to the DS processing when the DS unitdetermines that this slice name is already in a write locked state.

As illustrated by block 904, the DS unit may invoke write lock (e.g.,write in a local table for this slice name) for this slice name when theDS unit determines that this slice name is not already in a write lockedstate.

As illustrated by block 906, the DS unit stores the slice and revisionand sends a write confirmation command message to the DS processingwhere the message includes the write confirmation, the slice name, andthe revision.

As illustrated by block 908, the DS unit starts a rollback timer wherethe time value may be determined by the DS unit based on a predeterminedvalue (e.g., a lookup) or a variable value based in part on a systemperformance indicator. For example, a longer rollback timer may bedetermined when the system performance indicator indicates that thesystem is busier than the average.

As illustrated by block 910, the DS unit determines if the write commitcommand has been received from the DS processing when the rollback timeris active. As illustrated by block 912, the DS unit removes the writelock and makes the slice visible in subsequent retrievals when the DSunit determines that a receive commit was received while the rollbacktimer is active.

As illustrated by block 914, the DS unit determines if the rollbacktimer has expired when the DS unit determines that a receive commit hasnot been received while the rollback timer is active. The DS unitbranches back to the step of determining if a write commit command hasbeen received when the DS unit determines that the rollback timer hasnot expired.

As illustrated by block 916, the DS unit rolls back the version to theprevious slice version (e.g., subsequent retrievals will provide thelast version, not the current version), removes the write lock for thisslice name, and may delete the current version when the DS unitdetermines that the rollback timer has expired.

Generally, after a write the DS unit may either commit or rollback. Arollback may be implemented in response to a rollback request, or inresponse to a failure to receive a commit command. Both a commit and arollback result in the write lock for the slice being removed. In otherembodiments, the same behavior as receiving a rollback request, can beimplemented using an inactivity timer. In various embodiments, rollingback a request does not necessarily restore a slice to its previousversion, because in general the state of the latest slice is changedusing a commit procedure. Instead any temporary memory used for holdingan uncommitted slice is freed. Also, when a slice is committed, anyprevious revisions for that slice continue to exist.

FIG. 10 is another flowchart illustrating the storing of data where theDS unit stores a revision.

The method 1000 begins at block 1002, with the DS unit receiving a writecommand, slice name, slice, and revision from the DS processing. Asillustrated by block 1004, the DS unit determines a local timestampwhere in an embodiment the timestamp may be a UNIX time timestamp.

As illustrated by block 1006, the DS unit stores the slice, revision,and timestamp. Note that the DS unit may not delete previous revisionsof slices of the same data object such that the data object may besubsequently retrieved from previous revisions based in part on thetimestamp.

FIG. 11 is a flowchart illustrating the deleting of data where the DSunit processes a delete sequence for a revision.

The method 1100 begins at block 1102 with the DS unit receiving a deletecommand, slice name, and revision from the DS processing. In variousembodiments, delete commands may not be used; instead a write command isused to cause the DS unit to “write” a delete marker. As illustrated atblock 1104, the DS unit determines a local timestamp where in anembodiment the timestamp may be a UNIX time timestamp.

As illustrated as block 1106, the DS unit appends the timestamp and adelete marker to the slice. Note that the DS unit may not delete theslice in favor of marking when the delete command was received. Inanother embodiment, the DS unit deletes selective slices to free upmemory while preserving at least a read threshold number of slices perdata such that the data object may be subsequently retrieved fromprevious revisions based in part on the timestamp.

FIG. 12 is a flowchart illustrating the retrieving of data where the DSunit retrieves a particular revision of a data segment (e.g., of a dataobject) based in part on the timestamp. In other words, the DS unit willretrieve sluices of a snapshot of the data object.

The method 1200 begins at block 1202 with the DS unit receiving a readcommand, slice name, and timeframe from a requester (e.g., the DSprocessing unit, the DS managing unit, the storage integrity processingunit, or the user device). Note that the timeframe may or may not beexactly aligned with timestamps associated with previous revisions. Asillustrated by block 1204, the DS unit determines which local timestampis closest to the timeframe based on a comparison of timestamps to thetimeframe.

As illustrated by block 1206, the DS unit determines the slice for thetimestamp based on a lookup. Note that this slice revision representsthe snapshot closest to the received timeframe. As illustrated at block1208, the DS unit retrieves the slice and revision and sends the sliceand revision to the requester.

In some embodiments, a read request returns all available revisions,rather than a specific version or versions associated with a specifictime frame. When all revisions are returned in response to a readrequest, the DS processing unit can determine the best way to handle thevarious revisions received.

As may be used herein, the terms “substantially” and “approximately”provides an industry-accepted tolerance for its corresponding termand/or relativity between items. Such an industry-accepted toleranceranges from less than one percent to fifty percent and corresponds to,but is not limited to, component values, integrated circuit processvariations, temperature variations, rise and fall times, and/or thermalnoise. Such relativity between items ranges from a difference of a fewpercent to magnitude differences. As may also be used herein, theterm(s) “coupled to” and/or “coupling” and/or includes direct couplingbetween items and/or indirect coupling between items via an interveningitem (e.g., an item includes, but is not limited to, a component, anelement, a circuit, and/or a module) where, for indirect coupling, theintervening item does not modify the information of a signal but mayadjust its current level, voltage level, and/or power level. As mayfurther be used herein, inferred coupling (i.e., where one element iscoupled to another element by inference) includes direct and indirectcoupling between two items in the same manner as “coupled to”. As mayeven further be used herein, the term “operable to” indicates that anitem includes one or more of power connections, input(s), output(s),etc., to perform one or more its corresponding functions and may furtherinclude inferred coupling to one or more other items. As may stillfurther be used herein, the term “associated with”, includes directand/or indirect coupling of separate items and/or one item beingembedded within another item. As may be used herein, the term “comparesfavorably”, indicates that a comparison between two or more items,signals, etc., provides a desired relationship. For example, when thedesired relationship is that signal 1 has a greater magnitude thansignal 2, a favorable comparison may be achieved when the magnitude ofsignal 1 is greater than that of signal 2 or when the magnitude ofsignal 2 is less than that of signal 1.

The present invention has also been described above with the aid ofmethod steps illustrating the performance of specified functions andrelationships thereof. The boundaries and sequence of these functionalbuilding blocks and method steps have been arbitrarily defined hereinfor convenience of description. Alternate boundaries and sequences canbe defined so long as the specified functions and relationships areappropriately performed. Any such alternate boundaries or sequences arethus within the scope and spirit of the claimed invention.

The present invention has been described above with the aid offunctional building blocks illustrating the performance of certainsignificant functions. The boundaries of these functional buildingblocks have been arbitrarily defined for convenience of description.Alternate boundaries could be defined as long as the certain significantfunctions are appropriately performed. Similarly, flow diagram blocksmay also have been arbitrarily defined herein to illustrate certainsignificant functionality. To the extent used, the flow diagram blockboundaries and sequence could have been defined otherwise and stillperform the certain significant functionality. Such alternatedefinitions of both functional building blocks and flow diagram blocksand sequences are thus within the scope and spirit of the claimedinvention. One of average skill in the art will also recognize that thefunctional building blocks, and other illustrative blocks, modules andcomponents herein, can be implemented as illustrated or by discretecomponents, application specific integrated circuits, processorsexecuting appropriate software and the like or any combination thereof.

What is claimed is:
 1. A method comprising: generating a plurality ofencoded data slices, each of the plurality of encoded data slicesincluding different encoded versions of a data portion, the differentencoded versions encoded to prevent reconstruction of the data portionfrom a single encoded data slice but allow reconstruction of the dataportion using at least a threshold number of encoded data slices;determining revision numbers associated with respective data slices ofthe plurality of encoded data slices; associating revision numbers witheach of the plurality of encoded data slices; and transmitting at leasta write threshold number of the plurality of encoded data slices andassociated revision numbers to be stored in distributed storage unitsincluded in a distributed storage network.
 2. The method of claim 1,further comprising: determining selected distributed storage units of aplurality of distributed storage units to which each of the plurality ofencoded data slices is to be transmitted.
 3. The method of claim 2,further comprising: transmitting write commands associated with theplurality of encoded data slices to the selected distributed storageunits; and determining if write confirmations are received from at leasta write threshold number of the selected distributed storage units. 4.The method of claim 3, further comprising: sending a write commitcommand to the selected distributed storage units in response toreceiving the write confirmations from at least a write threshold numberof distributed storage units.
 5. The method of claim 4, furthercomprising: determining a current directory mapping data objects tolocations of encoded data slices generated from the data objects,wherein the current directory reflects write confirmations associatedwith storage of the plurality of encoded data slices; caching aninstance of the current directory; generating a plurality of encodedcache slices from the cached instance of the current directory; andtransmitting at least a write threshold number of the plurality of theencoded cache slices to selected distributed storage units.
 6. Themethod of claim 5, further comprising: determining whether at least athreshold number of write confirmations associated with the encodedcache slices have been received; in response to a favorabledetermination, sending a write commit command to the selecteddistributed storage units; and in response to an unfavorabledetermination, retransmitting at least some of the encoded cache slicesto different selected storage units.
 7. The method of claim 1, furthercomprising: determining the revision numbers based on at least one of atimestamp, a random number, a user vault identifier, a user identifier,a data object identifier, a hash of the data object, and a hash of thedata object identifier.
 8. The method of claim 1, further comprising:transmitting the at least a write threshold number of the plurality ofencoded data slices to be stored in a plurality of different pillars;and associating a same revision number with each of the plurality ofencoded data slices included in a same pillar.
 9. The method of claim 1,further comprising: associating different revision numbers withdifferent encoded data slices associated with a same data object.
 10. Amethod comprising: determining target distributed storage units fromwhich to retrieve encoded data slices including a revision number, eachof the encoded data slices generated from a common data portion andencoded to prevent reconstruction of the common data portion from asingle encoded data slice, but to allow reconstruction of the commondata portion using at least a threshold number of encoded data slices;receiving at least a read threshold number of the encoded data slices inresponse to sending a retrieve command to the target distributed storageunits; determining if the revision numbers included in the at least athreshold number of encoded data slices match each other; and inresponse to a favorable match, decoding the at least a read thresholdnumber of encoded data slices to recover the common data portion. 11.The method of claim 10, further comprising: in response to an at leastpartially unfavorable match, determining at least one differentdistributed storage unit from which to request encoded data slices. 12.The method of claim 11, further comprising: determining the at least onedifferent distributed storage unit based, at least in part, on which ofthe target distributed storage units provided encoded data slices withmatching revision numbers.
 13. The method of claim 10, furthercomprising: sending a retrieve command to a number of the targetdistributed storage units at least as great as a read threshold number,but less than a total number of encoded data slices generated from acommon data portion.
 14. A distributed storage processing unitcomprising: a grid module to generate a plurality of encoded dataslices, each of the plurality of encoded data slices including differentencoded versions of a data portion, the different encoded versionsencoded to prevent reconstruction of the data portion from a singleencoded data slice but allow reconstruction of the data portion using atleast a threshold number of encoded data slices; a processor todetermine revision numbers associated with respective data slices of theplurality of encoded data slices, and associate the revision numberswith each of the plurality of encoded data slices; and an interface totransmit at least a write threshold number of the plurality of encodeddata slices to be stored in distributed storage units included in adistributed storage network.
 15. The distributed storage processing unitof claim 14, further comprising: the processor further to determineselected distributed storage units of a plurality of distributed storageunits to which each of the plurality of encoded data slices is to betransmitted; the interface further to transmit write commands commandassociated with the plurality of encoded data slices to the selecteddistributed storage units; and
 16. The distributed storage processingunit of claim 15, further comprising: the processor further to determineif write confirmations are received from at least a write thresholdnumber of the selected distributed storage units; and the interfacefurther to send a write commit command to the selected distributedstorage units in response to receiving the write confirmations from atleast a write threshold number of distributed storage units.
 17. Thedistributed storage processing unit of claim 16, further comprising:determining a current directory mapping data objects to locations ofencoded data slices generated from the data objects, wherein the currentdirectory reflects write confirmations associated with storage of theplurality of encoded data slices; caching an instance of the currentdirectory; generating a plurality of encoded cache slices from thecached instance of the current directory; and transmitting at least awrite threshold number of the plurality of the encoded cache slices toparticular distributed storage units.
 18. The distributed storageprocessing unit of claim 17, further comprising: determining whether atleast a threshold number of write confirmations associated with theencoded cache slices have been received; in response to a favorabledetermination, sending a write commit command to the particulardistributed storage units; and in response to an unfavorabledetermination, retransmitting at least some of the encoded cache slicesto different particular storage units.
 19. A distributed storageprocessing unit comprising: a processor to determine target distributedstorage units from which to retrieve encoded data slices including arevision number, each of the encoded data slices generated from a commondata portion and encoded to prevent reconstruction of the common dataportion from a single encoded data slice, but to allow reconstruction ofthe common data portion using at least a threshold number of encodeddata slices; an interface to receive at least a read threshold number ofthe encoded data slices in response to sending a retrieve command to thetarget distributed storage units; the processor further to determine ifthe revision numbers included in the at least a threshold number ofencoded data slices match each other; and a grid module to decode the atleast a read threshold number of encoded data slices to recover thecommon data portion in response to a favorable match.
 20. Thedistributed storage processing unit of claim 19, further comprising: theprocessor further to determine at least one different distributedstorage unit from which to request encoded data slices, in response toan at least partially unfavorable match.
 21. The distributed storageprocessing unit of claim 20, further comprising: the processor furtherto determine the at least one different distributed storage unit based,at least in part, on which of the target distributed storage unitsprovided encoded data slices with matching revision numbers.
 22. Thedistributed storage processing unit of claim 19, further comprising: theprocessor further to send a retrieve command to a number of the targetdistributed storage units at least as great as a read threshold number,but less than a total number of encoded data slices generated from acommon data portion.
 23. The distributed storage processing unit ofclaim 19, further comprising the processor further to reconstruct thecommon data portion from encoded data slices having an earlier revisionnumber in response to the interface failing to receive a read thresholdnumber of a current revision number of encoded data slices.